The USGS is using a set of advanced imaging and analysis tools to study the rocks within the eastern Adirondacks of upstate New York. The goal of these studies is to gain a better understanding of the geology and mineral resources in the area.
Iron mining was common in the Eastern Adirondacks in the late 1800s and early 1900s, but the area also contains deposits of rare earth minerals. These elements are used in mobile devices, rechargeable batteries, super-magnets, solar panels and other advanced technologies. In some parts of the Adirondacks these minerals can be found in the discards or tailings from earlier iron and titanium mines.
A combination of geophysical, geochemical, mineralogical, geochronological and mapping approaches is being applied. Geophysical methods help with imaging rock types buried beneath vegetation, soil or other rocks while geochemical, mineralogical and mapping studies are used to determine how rare earth elements and other commodities are distributed within those rocks. Combining these results with geochronological data will help us to understand the tectonic and magmatic history of the region and how the ore deposits formed more than a billion years ago.
Geophysical data can used to image buried rocks from depths just beneath trees and grass to several miles beneath Earth’s surface. Some methods work via measurements of subtle variations in Earth’s magnetic field or gravitational pull. When rocks with different magnetic properties are juxtaposed next to each other, such as an iron-rich gabbro and a quartz sandstone, they generate subtle differences in Earth’s magnetic field that can be detected. In a similar manner, rocks with different densities can generate subtle but measurable variations in Earth’s gravitational pull.
Radiometric methods use special sensors made with sodium iodide or bismuth to detect and measure the energy of naturally occurring gamma particles. Once the energy spectrum is known, relative amounts of potassium, thorium and uranium in rocks within about 1 meter of Earth’s surface can be estimated.
In December 2015, the USGS contracted high-resolution airborne surveys using magnetic and radiometric methods to image rocks buried beneath vegetation, soil, and other rocks in an area west of Lake Champlain. These data are available at ScienceBase.gov.
The iron, titanium, and rare earth element (REE) minerals in the eastern Adirondacks are all components of “iron-oxide apatite” (IOA) mineral deposits. Samples from these deposits and mine tailings will be analyzed for geochemistry and mineralogy using tools such as a scanning electron microprobe (SEM) or a petrographic microscope. These data will help determine the distribution of rare earth elements and co-commodities in the area. Valuable heavy rare earth elements such as terbium and dysprosium (used in advanced electronics) are of particular interest, and have been detected in the region.
The ages of rocks in the study area are being determined by using the mineral zircon, which is often present in trace quantities. Zircon grains are particularly useful for dating studies because they are resistant to alteration by weathering processes, even over millions of years. Zircons can thus record multiple episodes of tectonic or magmatic activity. These episodes appear as layers within a single grain. For example, a zircon grain can have an igneous core (created during magmatism) that is surrounded by dark rims (reflecting rock metamorphism, which is often associated with tectonic events). A scanning electron microscope is used to image the zircon layers. The layers are then dated using a SHRIMP (sensitive high resolution ion microprobe) and applying U-Pb geochronology methods. The SHRIMP has an analytical spot size of 20-30 microns, about one-quarter of the diameter of a human hair.
From these efforts, the history of tectonism, magmatism, and ore deposit formation will be better understood. This in turn will help us understand geologic features and ore formation at other iron-oxide-apatite deposits worldwide.
Shah, A.K., 2016, Airborne Geophysical Surveys over the Eastern Adirondacks, New York State: U.S. Geological Survey data release, https://dx.doi.org/10.5066/F72R3PT0.
Aleinikoff, J.N., and Walsh, G.J., 2017, New evidence for late to post-Shawinigan intrusion of the Hawkeye Granite Gneiss and Lyon Mountain Granite Gneiss, Adirondack Highlands, NY: Geological Society of America Abstracts with Programs, Vol. 49, No. 2, doi:10.1130/abs/2017NE-290044.
Aleinikoff, J.N., and Walsh, G.J., 2016, Ages and origins of low-Ti magnetite deposits of the eastern Adirondacks: SHRIMP U-Pb and REE evidence from complexly zoned zircon: Geological Society of America Abstracts with Programs, Vol. 48, No. 2, doi:10.1130/abs/2016NE-272005.
Aleinikoff, J.N., and Walsh, G.J., 2016, SHRIMP U-Pb systematics of zircon from anorthosite, nelsonite, and ilmenite-magnetite ore at the Tahawus mine, Adirondack mountains, NY: Evidence for a younger episode of emplacement, Geological Society of America Abstracts with Programs, Vol. 48, No. 7 doi:10.1130/abs/2016AM-286511.
Aleinikoff, J.N., and Walsh, G.J., 2015, A new interpretation for the ages of Adirondack plutonism: Implications for the nature and timing of Mesoproterozoic deformation and metamorphism: Geological Society of America Abstracts with Programs, Vol. 47, No. 7, p. 722. View Aleinikoff 2015 GSA abstract.
Shah, A.K., Walsh, G., Taylor, R., Taylor, C., Aleinikoff, J., Klein, A., Regan, S., and Lupulescu, M., 2016, Geophysical, geochemical, and geological approaches to evaluating rare earth resources in the Eastern Adirondacks, upstate New York, Geological Society of America Abstracts with Programs, Vol. 48, No. 7, doi:10.1130/abs/2016AM-281761.
Taylor, R.D., Lowers, H.A., Goldfarb, R.J., and Marsh, E.E., 2016, Determining hydrothermal fluid source for gold deposits through geochemistry of monazite and xenotime crystals: 35th International Geological Congress, 27 August - 4 September 2016, Cape Town, South Africa, 2 p. View Ryan IGC abstract. [PDF file, 573 KB]
Walsh, G.J., Aleinikoff, J.N., and Ratcliffe, N.M., 2016, New U-Pb zircon ages and field studies support Shawinigan deformation in the eastern Adirondacks: Geological Society of America Abstracts with Programs, Vol. 48, No. 2, doi:10.1130/abs/2016NE-272135.
The USGS Mineral Resources Program is working with the USGS National Cooperative Geologic Mapping Program (NCGMP), the New York State Geological Survey (NYSGS) and the University of Massachusetts at Amherst to coordinate geophysical, geochemical and geological analyses.
Fisher, D.W., Isachsen, Y.W., and Rickard, L.V., 1970, Geologic map of New York State, 1970: 1:250,000. Consists of five sheets: Niagara, Finger Lakes, Hudson-Mohawk, Adirondack, and Lower Hudson: Map and Chart Series No. 15, 5 geologic bedrock maps, scale 1:250,000.
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Valley, P.M., Hanchar, J.M., and Whitehouse, M.J., 2011, New insights on the evolution of the Lyon Mountain Granite and associated Kiruna-type magnetite-apatite deposits, Adirondack Mountains, New York State: Geosphere, v. 7, no. 2, p. 357–389. doi: 10.1130/GES00624.1
Long, K.R., Van Gosen, B.S., Foley, N.K., and Cordier, Daniel, 2010, The principal rare earth elements deposits of the United States—A summary of domestic deposits and a global perspective: U.S. Geological Survey Scientific Investigations Report 2010–5220, 96 p., http://pubs.usgs.gov/sir/2010/5220/.
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Crustal Geophysics and Geochemistry Science Center